Symptoms of abnormal heart rhythms are generally referred to as cardiac arrhythmias, with an abnormally rapid rhythm being referred to as a tachycardia. The present invention is concerned with the treatment of tachycardias that are frequently caused by the presence of an "arrhythmogenic site" or "accessory atrioventricular pathway" close to the inner surface of the chambers of a heart or in pulmonary veins. The heart includes a number of normal pathways that are responsible for the propagation of electrical signals from the upper to lower chambers necessary for performing normal systole and diastole functions. The presence of arrhythmogenic site or accessory pathway can bypass or short circuit the normal pathway, potentially resulting in very rapid heart contractions, referred to here as tachycardias.
Treatment of tachycardias may be accomplished by a variety of approaches, including drugs, surgery, implantable pacemakers/defibrillators, and catheter ablation. While drugs may be the treatment of choice for many patients, they only mask the symptoms and do not cure the underlying causes. Implantable devices only correct the arrhythmia after it occurs. Surgical and catheter-based treatments, in contrast, will actually cure the problems, usually by ablating the abnormal arrhythmogenic tissue or accessory pathway responsible for the tachycardia. It is important for a physician to accurately steer the catheter to the exact site for ablation and fix the catheter steadily at that site. Once at the site, it is important for a physician to control the emission of energy to ablate the tissue within or around the heart.
Of particular interest to the present invention are radiofrequency (RF) ablation protocols that have been proven to be highly effective in tachycardia treatment while exposing a patient to minimal side effects and risks. Radiofrequency catheter ablation is generally performed after conducting an initial mapping study where the locations of the arrhythmogenic site and/or accessory pathway are determined. After a mapping study, an ablation catheter is usually introduced to the target heart chamber and is manipulated so that the ablation tip electrode lies exactly at the target tissue site. Radiofrequency energy or other suitable energy is then applied through the tip electrode to the cardiac tissue in order to ablate the tissue of arrhythmogenic site or the accessory pathway. By successfully destroying that tissue, the abnormal signal patterns responsible for the tachycardia may be eliminated.
Typically a conventional electrophysiology catheter has a tip portion that is to be inserted into a pulmonary vein for mapping and treating focal atrial fibrillations. To better monitor and control the distance of said catheter into the vein, it is preferable that a fixation means be incorporated onto the catheter shaft at a predetermined point. When the penetrating means contacts the ostium of the pulmonary vein, the catheter cannot advance any further inside the pulmonary vein and the distance of the distal portion inside the vein is determined. A major drawback for a conventional catheter without a fixation means at its distal portion is its difficulty for maintaining the tip portion in certain position; the effectiveness of mapping and ablation is significantly compromised.
The tip section of a catheter is referred to here as the portion of that catheter containing at least one electrode. For illustration purposes, a catheter utilized in the endocardial radiofrequency ablation is inserted into a major vein or artery, usually in the neck or groin area. The catheter is then guided into an appropriate chamber or pulmonary vein of the heart by appropriate manipulation through the vein or artery. The tip of a catheter must be manipulatable by a physician from the proximal end of the catheter, so that the electrodes at the tip section can be positioned against the tissue site to be mapped or ablated. The catheter must have a great deal of flexibility and steerability in order to guide the catheter to the target site of the heart. It must permit user manipulation of the tip even when the catheter body is in a curved and/or twisted configuration. The tip section of a conventional electrophysiology catheter that is deflectable usually contains one large electrode about 4 mm in length for ablation purpose.
After the exact location of a target tissue is identified, the ablation catheter may still not easily approach the target site. An external ultrasonic imaging capability therefore becomes in need so that ablation is not taking place in an inappropriate location. The fluoroscope time can be substantially cut short when an external ultrasonic imaging is used instead. In the U.S. Pat. No. 4,794,931, there has been disclosed a catheter and system which can be utilized for ultrasonic imaging. However, there is no disclosure to how such a catheter and system can be utilized in conjunction with an endocardial ablation catheter having a tip section with fixation means to achieve the desired ultrasonic imaging and ultimately the desired mapping or ablation.
While a radiofrequency electrophysiology procedure using an existing catheter has had promising results in pulmonary veins for focal atrial fibrillations, the tip section of a known catheter may not steadily stay in place for intimately contact the cardiac tissue during the mapping and/or ablation period. Therefore there is a need for an improved catheter which can be utilized for mapping and ablation and in which is possible to temporarily affix the tip section of the catheter so that it will remain in a desired position on the wall of the heart during beating of the heart.